Nest- no power to Rh with common connected


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Old 02-23-17, 04:48 PM
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Nest- no power to Rh with common connected

Hello! first time poster so please be gentle.

I recently bought a Nest for my home. Furnace is a Lennox 80UHG with a Surelight Integrated control board.

When I removed the old thermostat, the wires were connected to W1, Rh and G. The Nest would not stay charged, so I read enough to realize I would need a common wire. There were additional wires in the cable, but they were not hooked up at either end. So I opened the furnace, and connected the spare blue wire to the C terminal on the control board.

When I hook up the blue wire to the C terminal on the Nest base, I get the red symbol indicating no power to Rh...but if I remove the common, it all works fine

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Any ideas what could be causing this?

Thanks
 
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Old 02-23-17, 05:10 PM
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I would try moving the red wire to the Rc terminal.
 
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Old 02-23-17, 05:17 PM
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Yep, I tried that, in fact the original wiring was to the Rc terminal with a jumper to the Rh. Nest internally jumpers the Rc and Rh, so it made no difference.
 
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Old 02-23-17, 07:35 PM
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You either have a defective nest or the 24vac is sagging.
Use a meter to check from C to R at the board when the nest is connected and not connected.
 
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Old 02-24-17, 09:06 AM
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R-C (Nest connected) - 26.7v
R-C (Nest not connected) - 26.8v
 
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Old 02-24-17, 09:12 AM
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Sometimes the R and W get swapped and you get this error after adding a C. Verify wiring at all locations.
 
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Old 02-24-17, 09:26 AM
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The wire colours at the control board all match up with the base.

When I took the original thermostat off, the Rh and the W1 wires were the wrong way around. I did not know this at the time, so I hooked the Nest up the same way, and it worked fine (but no charging). Since reading about this, I noticed they were reversed, so I swapped them. The Nest still does the same thing, no matter which way round R and W are. It works, but does not charge, and connecting the C wire stops it working.
 
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Old 02-24-17, 10:23 PM
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You said that there were additional wires in the cable available. Maybe the blue wire is defective so try another.
 
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Old 02-24-17, 11:18 PM
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Based on the voltages you measured.... the stat is not charging.
I'd say the nest has a problem with it's charging circuit.
 
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Old 02-25-17, 05:53 AM
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I agree with PJ, it sounds like a bad stat.
 
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Old 02-25-17, 06:02 AM
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@Astuff...there are additional wires, I had tried the orange initially and am now on the blue, so it's not that.

@PJmax, Houston204, thanks, I will contact Nest about this.
 
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Old 03-15-17, 02:37 PM
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All fixed, Nest sent a replacement baseplate which seems to have fixed the issue...thanks for your help guys.
 
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Old 03-15-17, 03:32 PM
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Thanks for letting us know how you made it out.
It's especially helpful to others that have or will have the same problem.
 
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Old 03-15-17, 04:03 PM
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I've been the guy trying to solve a problem and finding dead threads...didn't want my thread to end up that way...

https://www.xkcd.com/979/
 
 

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